1. Field of the Invention
The present invention relates to a printing apparatus and printing control method, and in particular, to a printing apparatus including temperature sensing means for a print head, and a printing control method.
2. Description of the Related Art
A thermal ink jet printing method (hereinafter also referred to as an ink jet printing method) performs printing by energizing heaters to generate heat energy, thus generating bubbles in ink. In an inkjet printing apparatus, ink ejection amount (the volume of ink pushed out through nozzles) and ink ejection speed are significantly affected by the temperature of the ink (hereinafter also referred to as the ink temperature) near the heaters. An increase in ink temperature increases the ink ejection amount and the ink ejection speed. On the other hand, a decrease in ink temperature reduces the ink ejection amount and the ink ejection speed.
A change in ink ejection amount changes the density of an output image. Furthermore, a change in ink ejection speed changes an ink attachment position on a print medium. As a result, a density distribution may occur in the printed image, thus degrading print quality.
Thus, for the ink jet printing apparatus, an ink ejection control technique for allowing ink to be ejected constantly with respect to the ink temperature serves to improve the print quality. For the constant ejection with respect to the ink temperature, it is important to control the temperature of a print head by accurately determining the ink temperature.
However, directly sensing the ink temperature is difficult. Hence, the temperature of a print head board (hereinafter also referred to as the print head temperature) is commonly sensed so that ink ejection control and print head thermoregulation control are performed based on the sensed temperature. As a sensor for sensing the print head temperature, a diode sensor is often used which is formed on the same silicon chip on which ejection heaters are formed. This is because the diode sensor is manufactured by film formation and thus requires reduced costs and because the diode sensor is formed on an Si substrate with a high heat conductivity and thus offers high responsiveness.
The diode sensor does not involve a significant manufacturing variation in a proportionality coefficient (hereinafter also referred to as an inclination) for the temperature and output voltage. However, the diode sensor involves a great variation in output voltage value at a constant temperature (hereinafter also referred to as a zero intercept or an offset). Thus, maintaining the variation within an allowable range during actual use is relatively difficult. Consequently, processing may be executed to calibrate the offset. For example, the following are stored: a temperature (Tdef) corresponding to a voltage value obtained if the temperature of the print head has not been increased and is equivalent to the room temperature, and the room temperature (Tr) obtained by a thermistor in the printing apparatus main body. The offset value Tadj of the diode sensor in the print head is:Tadj=Tr−Tdef. 
Hence, given that the temperature corresponding to the voltage value of the head diode sensor in a certain state is Tdi, the print head temperature (Th) can be obtained by:Th=Tdi+Tadj. 
However, if the print head with the increased temperature is re-replaced (the print head is removed from the printing apparatus and then re-installed therein) or a user performs the operation of repeatedly powering on and off the printing apparatus main body, the temperature of the print head may be higher than the room temperature. In this case, the temperature Tdef, which corresponds to the reference temperature of the head indicated by the diode sensor, may be set to an incorrect value.
For example, Japanese Patent Application Laid-Open No. H07-209031 (1995) discloses a technique to obtain a more accurate offset value by updating the print head temperature a predetermined time after the calibration of the temperature during power-on or head installation.
However, in the technique disclosed in Japanese Patent Application Laid-Open No. H07-209031 (1995), if there is a difference between the actual temperature of the print head and the environmental temperature of the printing apparatus immediately after power-on or head installation, an incorrect correction value may be acquired. In particular, if the actual temperature of the print head is higher than the environmental temperature of the printing apparatus as in the above-described example, correction may be performed based on a numerical value deviating from the accurate correction value. For example, in case that correction is performed when the actual temperature of the print head is higher than the environmental temperature of the printing apparatus, the printing apparatus determines that the print head temperature is low, though the temperature is actually high. Thus, continuing a printing operation may excessively increase the print head temperature.
The present invention has been developed in view of the above-described problems. An object of the present invention is to provide a printing apparatus and a printing control method which allow the temperature sensor for the print head to be quickly and accurately calibrated even if the print head is assumed to have been replaced, for example, immediately after power-on or head installation.